As one student I've asked said, if we manage to keep cars out of town, air pollution will considerably diminish.

diminish 是动词，意思是"减少、降低"。这个词由前缀 di-（表示"向下"）和词根 minish（来自拉丁语 minuere，表示"减少"）组成，字面意思是"使变小、使减少"。在句中表示空气污染程度会明显下降。